Method and apparatus for pumping liquid from wells

ABSTRACT

An improved, unattended, liquid pumping device for oil and gas wells featuring a bellows controlled flow valve that opens and closes at preset pressures. The liquid pumping device may operate within a liner suspended into the well. The liquid pumping device may also include sealing elements configured to deform radially to allow the pumping device to pass through restrictions in the well or liner.

FIELD OF THE INVENTION

The invention relates to an autonomous pressure actuated liquid pump for use in gas and oil wells. In particular, it relates to liquid lift pumps in which well pressure, acting against an internal bellows or diaphragm, causes an internal flow control valve to open or close thereby releasing or shutting-in well gas flow. More particularly, the present invention relates to the use of such pumps with a liner that is inserted into open hole and large diameter gas and oil wells.

BACKGROUND

The economic viability of marginal petroleum wells, often referred to as stripper wells, depends on the well's product flow and pressure capacity and the rate at which undesirable liquids (i.e., brine) infiltrate the well. Removing liquid from gas or oil stripper wells has been accomplished with pump jacks or casing plungers in cased wells, while siphon strings or tubing plungers have been used in small diameter tubing. However, economic viability has restricted most applications to passive liquid removal techniques.

In large diameter casings (3″ or larger), casing plungers, such as the one disclosed in U.S. Pat. 6,851,480 have had some success in improving the economic viability of stripper wells. However, variations in ID, at each new section of casing, or at joints, or at collars, cause a potential stopping point for a plunger descending a well. These variations in the ID also creates pressure leakage areas that could potentially stop a plunger on its upward travel. Interconnect discontinuities also provide natural trapping areas for well contaminants, such as salt rings, that worsen the descending and ascending plunger problems.

Due to the problems associated with ID variations in a well, casing plungers may become stuck in the well. Bringing a casing plunger to the surface when it is stuck in the well may require venting the well to dump tanks to provide additional pressure differentials. Often lubricants and salt dissolving chemicals are added to free stuck plungers and at times retrieval gear is brought in to fish-out stuck plungers. All of these efforts to retrieve a stuck casing plunger increase the downtime of the wells and increase well tender man-hours.

Many mechanical devices have been proposed for accomplishing the sealing of casing and tubing plungers used in 2″ inside diameter and larger applications. Petro-chemical well environments however are quite corrosive and contaminating for mechanical linkages and small moving parts. Thus the industry has standardized on molded, flexible, friction cups to provide the sealing function in full size well casings and tubing, while brushes rather than cups are sometimes used for small diameter tubing applications they are not applicable to larger diameter configurations. Several different implementations of flexible cups are being used but they are not designed for operating in tube configurations in which the inside diameter is reduced 5 to 20% in section-to-section couplers and well head connectors.

SUMMARY OF THE INVENTION

This invention provides a self-actuating solution to overcome the shortcomings of the prior art devices. According to one aspect, the invention provides 10 a system for pumping fluid from a well. The system includes an elongated hollow tubing insertable into the casing of a well, and a casing plunger operable within the hollow tubing. A seal may also be provided to form a fluid tight seal between the liner and the casing of the well.

According to another aspect, the invention provides a method for pumping fluid from a well. The method includes the step of suspending a liner into the casing of a well. A pump is placed into the liner such that the pump descends through the liner into the liquid. The pump is automatically closed in response to a pressure differential at the pump, such that the pump forms a fluid seal to substantially impede fluid flow around the pump in the liner. The pump is raised along with a column of fluid in the liner in response to a build up of fluid pressure from closing the pump. The pump is then automatically opened in response to a decrease in pressure below the pump relative to the pressure above the pump.

According to yet another aspect, the invention provides an improved seal for a pump for pumping fluid from a well. The seal forms a fluid-tight seal between the pump housing and an internal surface of the well. The seal comprises first and second sealing elements that are axially spaced apart from one another, and which project radially outwardly. A fluid passage extends through the pump housing, having an inlet port below the seal and a discharge port above the seal. A pressure sensitive valve disposed within the fluid passage automatically closes when a pressure on the valve system is greater than a closing pressure, and automatically opens when the pressure on the valve system is less than an opening pressure.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a view of a well-pumping system according to the present invention;

FIG. 2 is a cross sectional view of the pumping device of the well-pumping system illustrated in FIG. 1;

FIG. 3 is an enlarged cross-sectional view of the well head of the will pumping system illustrated in FIG. 1;

FIG. 4 is an enlarged cross-sectional view of a tail stop of the well-pumping system illustrated in FIG. 1;

FIG. 5 is an enlarged fragmentary cross-sectional view of a connector for a liner in the well-pumping system illustrated in FIG. 1;

FIG. 6 is an enlarged cross-sectional view of sealing elements of the pumping device illustrated in FIG. 2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIGS. 1 and 2, a system for pumping liquid out of a well is illustrated. The system preferably includes a liner 30 that hangs from the top of a well 5. A pump in the form of a casing plunger 60 forms a fluid-tight seal with the interior wall of the liner 30.

To pump fluid out of the well, the liner 30 is lowered into the casing of the well. The liner 30 is fed into the well until the bottom end of the liner sinks into the fluid in the well. The pump is then inserted into the liner and lowered through the liner, so that it sinks into fluid in the well. Once the fluid pressure in the liner above the pump exceeds a threshold, the pump 60 seals-off the liner. In doing so, the device seals in the gas in the well, causing the fluid pressure below the pump to build up. The fluid pressure below the pump then drives the pump upwardly along with the fluid above the pump. As the pump 60 is driven upwardly, the fluid above the pump is discharged through discharge lines 19, 21 connected to the well 5. When the pump reaches the top of the well, the gas pressure below the pump drives the pump out of the liner and into a receiver 15 that maintains the pump above the lower discharge line 21 while gas from the well flows from the liner and through the lower discharge line. When the flow of gas from the well diminishes, the pump 60 is again lowered through the liner to pump more fluid out of the well.

The raising and lowering of the pump 60 is controlled automatically in response to the fluid pressure in the well. Specifically, the pump 60 includes a valve 70 that controls the flow of fluid through the pump. A biasing element 80 controls the operation of the valve 70. More specifically, the biasing element 80 biases the valve 70 into an open position. When the valve 70 is open, the pump 60 descends into the well, and fluid flows through the pump. The rate of descent is limited by the friction between the pump and the well walls and flow restrictions through the pump. When the pump reaches the liquid level in the liner 80, it continues to descend, but at a reduced rate.

When the pressure differential across the pump 60 exceeds a threshold (closing threshold) related to the biasing force of the biasing element, the valve 70 automatically closes so that fluid can no longer flow through the pump. As described above, the fluid pressure in the well builds up and then drives the pump upwardly. At the top of the well 5 the pump 60 is displaced into a receiver assembly 60 that maintains the pump. While the pump is maintained in the receiver, the gas pressure in the well dissipates as gas flows through the lower discharge line 21. When the fluid pressure across the pump drops below a threshold (opening threshold), the biasing element 80 automatically opens the valve 70 and the pump 60 descends again into the well. In this way, the pump automatically descends and ascends within the well to pump fluid from the well.

Referring to FIG. 3, the liner 30 will be described in greater detail. The liner 30 need not be used in each system in which the pump 60 is used. However, in certain applications the well is not suited for use with a pump, such as a casing plunger. For instance, the well may be an open hole well that only has a casing for the first few hundred feet of the well, therefore, most of the well does not have a casing that the pump can seal against. Alternatively, the casing diameter may be too large or lack pressure integrity for the operation of the pump. In such instances, the liner 30 may be incorporated.

The liner 30 is an elongated hollow tubing. The liner may be a single length of tubing, however, preferably the tubing is comprised of a plurality of separate sections that are connected together. Accordingly, preferably each section of liner has fittings 32 at each end to connect the different sections using couplers 38. As discussed further below, packing material 25 is used to maintain a fluid-tight seal between the well casing and the liner. If a section of tubing needs to be added to the tubing, the packer seal 25 is relaxed so that the tubing coupler 38 can pass into the well casing. The tubing coupler is then repositioned around the tubing 30.

Referring to FIG. 5, the fittings are illustrated in greater detail. The fitting provides a sufficient seal to maintain the pressure build up within the liner during use of the device. In addition, the fitting 32 provides a connection having sufficient tensile strength to support the load of the liner. Specifically, since the liner may need to extend thousands of feet into the well, the weight of the liner becomes significant. In the present instance, the approximately 4000 feet of the liner weighs approximately 5,000 pounds.

The fitting 32 comprises an external ferrule 33 and an insert 34. The liner 30 is disposed between the ferrule 33 and the insert 34. The ferrule 33 is then crimped down onto the insert to fixedly attach the liner 30 to the fitting 32. As shown in FIG. 5, the end of insert 34 flares outwardly forming an externally threaded portion 35. The threaded portions are used to connect the sections of the tubing. More specifically, a collar 38 (shown in FIG. 4) having internal threads engages the threaded portions 35 of fittings from two sections to connect the two sections.

The tubing may be formed of a variety of materials, including metal or plastic. Preferably, the conduit is configured so that the conduit can be spooled and shipped on large reels (8 to 12 feet in diameter) prior to insertion into the well. Although a variety of plastic tubing can be utilized, in the present instance, the plastic tubing is a polymer plastic including fiber bands and tension straps to support the weight of the tubing and the operational pressures.

The liner 30 extends into the well so that the liner is within the casing 10. The liner may extends down into the well beyond the casing or in some applications, the liner may terminate within the casing. The liner is connected to the well at the well head in such a manner as to provide a fluid-tight seal between the casing 10 and the liner 30.

Referring to FIG. 3, the connection of the liner to the well head is illustrated in greater detail. The fitting 32 at the top end of the tubing threadedly engages a hanger 23 having a mounting flange 24 on the well head. The mounting flange cooperates with a flange 17 on the receiver 15 to attach the receiver to the well head. Packing material or a packer 25 is positioned between the liner 30 and the casing 10 to seal off the annular space between the interior wall of the casing 10 and external wall of the liner 10. The packer 25 provides a fluid-tight seal between the casing and the liner 30 that is able to maintain the seal under the fluid pressures created during operation of the pump 60. In this way, when the casing plunger 60 seals off the liner to shut in the well, the packer 25 prevents gas or other fluid from leaking out of the well between the casing and the liner. The packer 25 may be formed from any of a number of materials, such as elastomeric polymer.

Tubing sections are added to the liner until the lower end of the tubing extends into the fluid in the well. Typically, the casing includes a perforated section toward the bottom of the well. The perforations are holes through the sidewalls in the casing that allow the marketable fluid(s) in the ground to pass through the casing and then up the well. However, undesirable fluid, such as brine may accumulate in the well and extend above the perforated section, thereby reducing the delivery of desired fluid(s). The pump 60 may be used to remove the undesired fluid so that the level of such fluids is below the perforations thereby improving the flow of the marketable fluid(s). Accordingly, it is desirable to extend the tubing sections into the well so that the bottom end of the liner 30 extends to, or below the level of the perforations in the casing. In this way the pump can be used to pump fluid out of the well at the level of the perforations. In other words, the liner may be positioned so that the pump descends through the liner to a point at or below the perforations, and then automatically closes to lift the fluid to the surface as described previously. By doing so, the pump may lift sufficient undesirable fluid to reduce the fluid level to a level below some or all of the perforations in the perforated section.

A tail cap 40 attached to the lower end of the tubing 30 prevents the casing plunger from descending out of the liner in the event that the casing plunger does not close before reaching the bottom of the liner. The tail cap 40 comprises an externally threaded portion that is connected to the tubing 30 with a coupler 38, similar to the manner in which sections of the tubing are connected. The tail cap 40 comprises a plurality of orifices 42 that allow fluid to flow from the well 5 into the liner 30. Additionally, a bumper stop 45 in the tail cap 40 operates as a cushion to decelerate the casing plunger 60 as the casing plunger bottoms out on the tail cap. The bumper stop comprises a rod 47 having a flared head, and a biasing element, such as a compression spring 48. The rod and spring are disposed within a cylindrical housing 49 that fits over a cylindrical nipple 44 formed on the inside of the tail cap 40.

Referring now to FIG. 2, the details of the pump 60 will be described in greater detail. The pump 60 includes an elongated substantially hollow cylindrical housing 62. A lower housing 65 is fixedly attached to the lower end of the cylindrical housing 62. An end cap 75 closes the lower end of the lower housing. Preferably, the lower end cap 75 is releasably connected with the lower housing 65. In the present instance the lower end cap 75 is threadedly connected to the lower housing. A plurality of holes in the lower end cap 75 form inlet ports 76, so that fluid can flow into the pump 60 through the inlet ports 76 when the pump descends into the well.

A top cap 90 is attached to the upper end of the housing 62. The top 90 has a central bore providing a fluid path. The lower end of the top cap 90 is attached to the upper end of the housing 62. Preferably the top cap 90 is releasably connected to the housing; and in the present instance, the top cap 90 has external threads that mate with internal threads in the housing 62 to attach the top cap to the housing.

The upper end of the top cap 90 is generally open, and preferably includes an internally threaded portion for mounting a stem 68. The stem 68 preferably has an externally threaded portion cooperable with the top cap 90 to releasably attach the stem to the top cap. In this way, the stem threads into the top cap thereby sealing the upper end of the top cap.

As shown in FIG. 2, a plurality of holes through the sides of the top cap 90 provide outlet ports 92. In this way, fluid flowing through the pump 60 flows through the top cap 90 and out the outlet ports 92.

A plurality of sealing elements or cups 85, 86 disposed around the housing provide a fluid-tight seal between the housing and the inner wall of the well 5. The cups 85, 86 are disposed between the inlet ports 76 at the bottom of the pump 60 and the outlet ports 92 at the top of the pump. The cups 85, 86 are elastomeric elements having a central bore. The cups 85, 86 are spaced apart axially from one another by a spacer 88. The spacer 88 is an elongated cylindrical collar having an internal diameter slightly larger than the external diameter of the housing.

The cups 85, 86 and spacer 88 are captured on the housing between a locking ring 95 and a lip that is the formed by the top edge of the lower housing 65. Specifically, an internal annular shoulder of the lower cup 86 abuts both the top edge of the lower housing, and the locking ring 95 threaded onto the top cap 90 engages the top edge of the upper cup 85.

The locking ring 95 is a threaded collar or nut that cooperates with external threads on the top cap 90. In this way, the locking ring 95 is operable to tighten down or compress the cups 85, 86. Since the cups 85, 86 are formed of elastomeric material, preferably a metal washer is disposed between the locking ring 95 and the upper cup 85. The metal interface between the locking ring and the washer facilitates turning the ring to tighten down on the cups 85, 86.

During use, the cups may wear and need to be replaced. Accordingly, preferably the pump 60 is configured so that the cups 85, 86 can be readily removed and replaced without disassembling the pump. Therefore, in the present instance the top cap 90 is preferably configured so that it need not be removed to replace the cups. Specifically, preferably the exterior diameter of the top cap 90 is small enough to allow the cups to slide over the top cap. In particular, preferably the external diameter of the top cap 90 is approximately the same as, or less than, the external diameter of the housing.

Referring now to FIG. 6, the lower cup 86 is illustrated in more detail. The lower cup 86 comprises a generally cylindrical body 102 and a pair of spaced apart seals 104, 106 in the form of circumferential flanges that project outwardly from the body. The body 102 and the seals 104, 106 may be formed of separate materials. However, in the present instance the body and the seals 104, 106 are a unitary piece of molded elastomeric material. The seal 104, 106 are formed of material that is resiliently deformable radially. In the present instance, each flange has an axial thickness “t” and a radial width “w”. The width “w” is greater than the thickness “t” and the width may be twice the thickness “t”.

The seals 104, 106 form a fluid-tight seal, sealingly engaging the casing of the well, or if a liner 30 is used. Additionally, the seals 104, 106 are configured to form a sliding seal with the interior wall of the casing or the liner, if used. Since the flanges 104, 106 are spaced apart, the flanges sequentially seal the well when the cups encounter a restriction in the path that the pump is traveling (i.e. the casing or the liner). For instance, as described above, the liner 30 may be formed of a number of sections connected together by fittings 32. As shown in FIG. 5, the insert 34 of the fittings 32 creates a restriction in the internal diameter of the liner. Preferably the fitting is relatively thin and only restricts the diameter minimally. Nonetheless, the fitting creates a restriction. As the casing plunger 60 travels upwardly after shutting in the well, the upper seal 104 engages the fitting 32 first, and deflects radially inwardly. The upper seal 104 is configured to maintain a seal with the liner even as the seal deflects radially inwardly. However, even if the restriction deflects the upper seal in such a way that releases a portion of the sealing engagement between the upper seal and the liner, the lower seal 106 maintains the seal with the liner to prevent leakage past the cup. Subsequently, as the lower seal 106 encounters the restriction, the upper seal 104 maintains the seal with the liner in the event that the lower seal loses sealing engagement with the liner.

As described above, the cups 85, 86 are configured to maintain a seal while passing through a constriction in either the casing or the liner, if one is used. Similarly, if a liner 30 is used, the wall of the liner may become out of round. For instance, when the liner is spooled onto a reel, the liner may flatten to become more of an oval or eccentric cross-section rather than a circular cross-section. The cups are formed so that the seals 104, 106 deform radially and/or axially to conform to the oval or eccentric cross-section of the liner. In this way, the cups are able to maintain a sliding fluid-tight seal with the out of round interior of the liner.

Configured as described above, the cups 85, 86 are able to maintain a sealing engagement with the casing or the liner if one is used, while being able to deflect sufficiently to allow the casing plunger to pass through a restriction in the liner or casing. More specifically, the cups are configured such that the seals deflect radially inwardly sufficiently to pass through a restriction of 5-20% of the diameter of the casing or liner.

Referring to FIG. 2, the valve 70 controls the flow of fluid through the housing 62. In FIG. 2, the valve 70 and biasing element 80 are shown in elevation. The valve 70 comprises a valve element 72 that cooperates with a valve seat 73 to form a fluid-tight seal. Preferably, the valve element 72 is formed of an elastomeric material. The valve seat 73 is preferably a tapered annular surface formed in the interior wall of the lower housing.

When the valve is closed, fluid does not flow through the pump. In addition, since the cups 85, 86 provide a fluid-tight seal between the housing 62 and the wall of the well 5, fluid does not flow around the pump. Accordingly, when the valve 70 is closed, the pump 60 operates as a seal, sealing the well closed. This allows a pressure differential to build up across the tool. Specifically, when the valve is closed, the pressure below the cups increases relative to the pressure above the cups.

The biasing element 80 is a pressurized bellows that bias the valve 70 toward an open position in which fluid can flow through the pump through the inlet and outlet ports 76, 92. The bellows 35 are operable to expand and contract vertically. The lower end of the canister is generally open, having an annular flange extending radially inwardly to form a lip.

The bias of the bellows 80 is controlled in part by the fluid pressure within the bellows. A cavity is formed within the bellows and a fill valve attached to the housing of the bellows controls the flow of fluid into the bellows. In this way, the bellows can be charged by filling the bellows with pressurized air through the fill valve. As the bellows are filled with pressurized air, the bellows expand outwardly, displacing the valve element 72 downwardly.

The bellows 80 compresses in response to hydrostatic pressure on the bellows when the pump is in the liquid in the well. As the bellows compresses, the valve 70 closes. The stroke of the valve element 72 between the opened position and the closed position corresponds to the compression of the bellows from the charged length to the compressed length when the valve 70 is closed.

Referring to FIG. 1, an upper discharge line 19 and lower discharge line 21 are connected to the well 5 for receiving the fluid from the well. The upper discharge line 19 extends between the well 5 and the lower discharge line 21. Preferably, the lower discharge line 21 is approximately twice as large in diameter as the upper discharge line 19. The opening from the well 5 to the upper discharge line 19 is vertically spaced along the well from the opening to the lower discharge line 21 a distance that is greater than the distance from the point that the lower cup 86 seals with the well to the point that the upper cup 85 seals with the well. In this way, when the device 60 is at the top of the well the upper and lower cups 85, 86 form seals with the walls of the receiver between the upper and lower discharge lines 19, 21. In this way, fluid from the well flows through the liner 30 and to the lower discharge line 21 while the pump 60 is disposed in the receiver.

A check valve 75 is disposed along the upper return line. The check valve 75 is configured to allow higher pressure fluid in the upper discharge line 70 to flow into the lower discharge line 80 and to impede fluid flow from the lower discharge line up into the upper discharge line. In this way, the fluid in the upper discharge line remains at a lower pressure than the fluid in the lower discharge line. An upper shut-off valve 72 is provided on the upper discharge line 70 to shut-off the upper discharge line, and a lower discharge valve 82 is provided to shut-off the lower discharge line. The shut-off valves 72, 82 may be any one of a number of types of valves, such as a ball valve.

When the friction cups 85, 86 pass above the lower discharge line 80, the shut-in well gas pressure discharges into the lower discharge line. The flow control valve 70 in the pump remains in the closed position as the shut-in pressure dissipates. The check valve 75 provides separation between the pressure above the pump (i.e. above the friction cups 85, 86) and the dissipating shut-in pressure in the lower discharge line 80, thereby maintaining a positive pressure differential across the pump 60. The gas pressure in the well is sufficient to support the pump to maintain it in the receiver until the valve 70 opens. As the fluid pressure in the well decreases below the preset pressure differential across the pump (from the high shut-in pressure), the flow control valve 70 opens. When the valve is opened, the pressure differential across the pump approaches zero and the pump descends into the well for additional liquid pumping.

It will be recognized by those skilled in the art that changes or modifications can be made to the above-described embodiments without department from the broad inventive concept of the invention. It should therefore be understood that this invention is not limited to the particular embodiments described herein but is intended to include all changes and modifications that are within the scope and spirit of the invention as set forth in the following claims. 

1. A method for pumping fluid from a well having a casing, comprising the steps of: suspending a liner through the casing and into liquid in the well; placing a pump in the liner such that the pump descends through the liner into the liquid; automatically closing the pump such that the pump forms a fluid seal to substantially impede fluid flow around the pump in the liner; raising the pump with a column of fluid in the liner in response to a build up of fluid pressure from closing the pump; and automatically opening the pump in response to a decrease in the difference between the fluid pressure above the pump and the fluid pressure below the pump.
 2. The method of claim 1 comprising the step of forming a fluid-tight seal between the liner and the casing to substantially impede the flow of fluid from the well between the liner and the casing.
 3. The method of claim 1 wherein the liner is a hollow tubing.
 4. The method of claim 3 wherein the step of suspending a liner comprises the steps of: inserting a first length of tubing into the well; connecting a second length of tubing to the first length of tubing; and inserting the second length of tubing into the well.
 5. The method of claim 1 wherein the step of suspending a liner comprises the step of unwinding the liner from a spool and inserting the liner into the well.
 6. The method of claim 1 comprising the step of attaching a cap to the bottom end of the liner, wherein the cap is operable to impede the displacement of the pump to prevent the pump from descending out of the bottom of the liner.
 7. The method of claim 6 comprising the step of cushioning impact between the pump and the cap.
 8. The method of claim 1 comprising the steps of: attaching upper and lower discharge lines to the well; and suspending the pump between upper and lower discharge lines after the step of raising the pump and before the step of automatically opening the pump.
 9. The method of claim 1 comprising the step of selecting a liner having a diameter of approximately 3″ or greater.
 10. A system for pumping fluid from a well having a casing, comprising: an elongated hollow tubing insertable into the casing of a well, wherein the hollow tubing has an interior surface; a casing plunger, comprising: a housing having an upper end and a lower end; a seal disposed intermediate the upper end and lower end, forming a fluid-tight seal between the housing and the interior surface of the tubing; a fluid passage extending through the case, having an inlet port below the seal and a discharge port above the seal; and a pressure sensitive valve system disposed within the fluid passage, such that the valve system automatically closes when a pressure on the valve system is greater than a closing pressure, and opens when the pressure on the valve system is less than an opening pressure; and a seal forming a fluid tight seal between the liner and the casing of the well.
 11. The device of claim 10 wherein the valve system seals the fluid passage when the valve system is closed, thereby substantially impeding fluid flow through device when the valve system is closed.
 12. The device of claim 10 wherein the tubing is spooled onto a reel.
 13. The device of claim 10 wherein the tubing comprises a plurality of separate sections and each section comprises a connector for connecting the sections.
 14. The device of claim 10 comprising a stop operable to impede displacement of the casing plunger beyond a bottom edge of the tubing.
 15. The device of claim 14 comprising a cushioning element operable to cushion the casing plunger as the casing plunger contacts the stop.
 16. The device of claim 14 wherein the stop comprises a cap enclosing the bottom edge, wherein the cap comprises a plurality of orifices.
 17. A pump for pumping fluid from a well having a generally cylindrical tube having an internal surface, wherein the pump comprises: a housing having an upper end and a lower end; a seal disposed intermediate the upper end and lower end, forming a fluid-tight seal between the housing and the internal surface of the cylindrical tube, wherein the seal comprises: a body; a first seal element projecting radially outwardly away from the body; and a second seal element projecting radially outwardly from the body, axially spaced from the first seal element; a fluid passage extending through the housing, having an inlet port below the seal and a discharge port above the seal; and a pressure sensitive valve system disposed within the fluid passage, such that the valve system automatically closes when a pressure on the valve system is greater than a closing pressure, and opens when the pressure on the valve system is less than an opening pressure.
 18. The pump of claim 17 wherein the seal is configured to maintain a sealing relationship with the internal surface as the seal passes through a restriction in the tube of 5-20% of the diameter of the tube.
 19. The pump of claim 17 wherein the tube has a portion having a cross section that is out of round, and wherein the seal is configured to maintain a sealing relationship with the internal surface of the tube when the pump passes through the out of round portion.
 20. The pump of claim 17 wherein the first and second seal elements each have an axial length and a radial thickness, wherein the radial thickness is greater than the axial length.
 21. The pump of claim 17 wherein the pump comprises a second seal configured similarly to the seal, axially spaced apart from the seal.
 22. The pump of claim 17 wherein the first and second sealing elements are configured to deform radially and axially.
 23. The pump of claim 17 wherein the first sealing element is operable to deform axially an amount greater than the axial thickness of the first sealing element.
 24. The pump of claim 17 wherein the first sealing element is a circumferential flange projecting outwardly from the body and the second sealing element is a circumferential flange projecting outwardly from the body. 